A wind turbine for an offshore wind park must be mounted securely to withstand the forces exerted on it by water and wind. In shallow offshore locations, the wind turbine tower can be mounted on a concrete plinth or foundation that extends into the sea bed. However, for other offshore locations, such mounting techniques may be unsuitable for various reasons. Usually, a “jacket structure” made of three or four upright steel pipes or legs is used to support the wind turbine tower in such a deep offshore location. The upright legs are interconnected by a mesh of braces in the manner of a pylon. The relatively small surface area of such a jacket structure means that it can withstand the forces exerted on it by the water in which it is submerged. The jacket structure is usually relatively narrow at the top, becoming progressively wider towards the base to provide the necessary structural stability. The base of the jacket structure can be anchored to a concrete foundation on the sea bed, or each leg of the jacket structure could be secured to a separate pile driven into the sea bed.
A jacket structure for an off-shore wind turbine is dimensioned so that about two thirds are submerged. For an average water depth of about 45 m, a jacket structure can therefore have a height of about 55 m-56 m. The footprint of such a jacket structure can be in the range of 20 m across. Legs and braces of such a jacket structure are generally made of steel pipes or pipe sections. For the necessary structural stability, the legs of such a jacket structure might be about 1.0 m-1.5 m in diameter, while a brace can have a diameter of about 0.6 m-0.8 m. It will be clear that legs and braces with such dimensions are heavy elements.
A conventional approach of assembling an underwater jacket structure involves assembling modules corresponding to layers of the jacket structure. Each module comprises identical leg sections interconnected by braces extending between the neighbouring leg sections. For a tapered jacket structure that is wider at the bottom and narrower at the top, a series of differently-sized modules is constructed, and the completed modules are then successively hoisted into position, stacked on top of each other, and fastened together. Assembly can be carried out in any appropriate location, for example close to a port or harbour. The finished jacket structure can then be skidded to a quayside for transfer to an offshore location.
A significant disadvantage of the known method is that the modular assembly is quite time-consuming. The layers must be assembled one after the other, since the stack must be built up starting from the lowest level. Also, workers are required to manipulate the heavy leg segments and braces into place during the assembly. Even though the heavy parts can be hoisted into place using a crane, workers are still placed at considerable risk. Altogether, the modular assembly is time-consuming and therefore comprises a significant portion of the overall cost of manufacture of a jacket structure. For an offshore wind park comprising many turbines, the costs of such an assembly procedure can be prohibitive.